As a process for preparing 4-hydroxybutyl acrylate, there has been known a process for preparing 4-hydroxybutyl acrylate, which includes the esterification reaction through dehydration of 1,4-butanediol and acrylic acid with a catalyst such as sulfonic acid or para-toluene sulfonic acid (see, for example, Patent Literature 1). However, the above-mentioned process has some defects such that the amount of by-products is great, that the process necessitates a complicated neutralizing step because an acid is used as a catalyst, and that a large amount of salts generated in the neutralizing step become wastes. In addition, the above-mentioned process has some defects such that unreacted acrylic acid remains in the resulting 4-hydroxybutyl acrylate in an amount which exerts baneful influence for using the 4-hydroxybutyl acrylate as a raw material for pressure sensitive additives for medical uses, coating agents for electronic materials and the like.
As a method for solving the defects in the above-mentioned process, there have been proposed a method for preparing a hydroxyalkyl monoacrylate, which includes the esterification reaction of an acrylic acid derivative with an alkane diol with a stannoxane compound as a reaction catalyst, to give a hydroxyalkyl monoacrylate (see, for example, Patent Literature 2), a method for preparing 4-hydroxybutyl acrylate, which includes transesterification reaction of 1,4-butanediol and methyl methacrylate in the presence of a stannoxane catalyst as a reaction catalyst, to give 4-hydroxybutyl acrylate (see, for example, column [0020] of Patent Literature 3 and columns [0033]-[0034] of Patent Literature 4), and the like. However, acid components such as acrylic acid remain in acrylates obtained in these methods in an amount which imparts stimulation to a skin, and which would cause corrosion. Therefore, it cannot be said that the acrylate is suitable for a raw material of pressure sensitive additives for medical uses, coating agents for electronic materials and the like.
As a method for removing a free carboxylic acid contained in the resulting hydroxyalkyl acrylate, there has been proposed a method which includes carrying out the transesterification reaction of an alkane diol with an acrylate with a distannoxane compound as a reaction catalyst, and contacting a solution of an organic solvent containing the resulting hydroxyalkyl acrylate and a free carboxylic acid with an alkaline aqueous solution (see, for example, Patent Literature 5). However, there are some defects in the above-mentioned method, such that productivity of the hydroxyalkyl acrylate is low, because the method necessitates an alkaline aqueous solution which hydrolyzes the hydroxyalkyl acrylate.
Also, it has been known to use a dialkyltin oxide such as dibutylthin oxide or di-tert-butyltin oxide as a catalyst for transesterification (see, for example, column [0004] of Patent Literature 6 and claim 1 of Patent Literature 7). The dialkyltin oxide is an attractive catalyst, because the dialkyltin oxide is excellent in catalytic activity even when its amount is mall, and is relatively inexpensive. However, it is considered that it is difficult to use the dialkyltin oxide as a catalyst, because the dialkyltin oxide is difficultly separated by an extraction operation when 4-hydroxybutyl acrylate is prepared by the transesterification reaction of 1,4-butanediol and methyl acrylate (see, for example column [0079] of Patent Literature 2 and column [0007] of Patent Literature 6). In addition, disproportionation reaction of the resulting 4-hydroxybutyl acrylate represented by the formula:
[Chem. 1]CH2═CHCOO(CH2)4OH→CH2═CHCOO(CH2)4OCOCH═CH2+HO(CH2)4OH
occurs due to the alkyltin oxide contained in the 4-hydroxybutyl acrylate, to generate 1,4-butanediol diacrylate as a by-product. This by-product, 1,4-butanediol diacrylate has a boiling point which is near to the boiling point of 4-hydroxybutyl acrylate. In addition, self-polymerization of 1,4-butanediol diacrylate easily occurs due to the presence of the alkyltin oxide as a catalyst. Moreover, a crosslinking reaction of 1,4-butanediol diacrylate easily occurs, because the 1,4-butanediol diacrylate has two acrylate groups in its molecule. Therefore, it has been thought that the separation of the by-product, 1,4-butanediol diacrylate is considerably difficult by a purification method such as distillation.
Also, when a hydroxyalkyl acrylate is prepared by a transesterification method, methanol is distilled off, and contains not only n-hexane which is an azeotropic solvent but also methyl acrylate which is a raw material. Therefore, there has been proposed that water is added to this methanol, and the resulting aqueous methanol solution is separated from the starting methyl acrylate and n-hexane by liquid-liquid separation, to collect the starting methyl acrylate (see, for example, Patent Literature 8). However, water cannot be easily separated from the aqueous methanol solution. Therefore, the aqueous methanol solution is currently discarded by incineration, nevertheless the methanol contained in the aqueous methanol solution is valuable. When the above-mentioned operation for treatment is carried out, there arise some defects such that a special incinerator such as an incinerator of waste water for incinerating the aqueous methanol solution is necessitated, and that running cost increases because a large amount of energy is required for incinerating the aqueous methanol solution.
In addition, as a method for efficiently separating methanol from the mixture of methyl(meth)acrylate and methanol with a reactor equipped with a distillation column, there has been proposed a method which includes using an azeotropic solvent which forms an azeotropic mixture together with methanol, recirculating a part of a condensate made of vapor which is distilled from the overhead of the distillation column, separating the residual condensate into two layers, supplying the upper layer of the two layers to the middle portion of the distillation column, taking out the lower layer from the distillation column, and collecting methyl(meth)acrylate from the bottom of the reactor (see, for example, Patent Literature 9). According to the above-mentioned method, methyl(meth)acrylate which exists in the bottom portion of the reactor can be collected. However, after the collection of the methyl(meth)acrylate, the methyl(meth)acrylate remaining inside the reactor cannot be collected.
Also, as a process for preparing an objective(meth)acrylate by the transesterification reaction of an alkyl(meth)acrylate used as a raw material and an alcohol, there has been proposed a process for preparing a (meth)acrylate, which includes carrying out a transesterification reaction in the presence of an azeotropic solvent which forms an azeotropic mixture together with an alkyl alcohol which is formed as a by-product, removing the alkyl alcohol which is formed as a by-product together with the azeotropic solvent from the discharge port positioning at the upper portion of the distillation column, controlling the temperature of the vapor discharging from the discharge port to a temperature not lower than the azeotropic temperature of the alkyl alcohol which is generated as a by-product and an azeotropic solvent which forms an azeotropic mixture together with the alkyl alcohol, and not higher than the temperature which is 2° C. higher than the azeotropic temperature, and controlling the temperature of the bottom of the distillation column to a temperature which is 10° C. lower than the boiling point of the azeotropic solvent or more and not higher than the boiling point of the azeotropic solvent (see, for example, Patent Literature 10). According to the above-mentioned process for preparing the (meth)acrylate, it is thought that a side reaction, Michael addition reaction can be suppressed, and a (meth)acrylate can be prepared in high productivity. However, after the preparation of an objective(meth)acrylate, the alkyl(meth)acrylate used as a raw material remains in the reaction system, and a method for efficiently collecting the remaining alkyl(meth)acrylate which is a raw material has not yet been examined.
Accordingly, in a process for preparing a (meth)acrylate which is prepared by a transesterification method with methyl(meth)acrylate used as a raw material, there has been desired to develop a process for preparing a (meth)acrylate by using a transesterification method, which enables the collection of the (meth)acrylate which is a raw material remaining in the reaction system in high efficiency.
Also, when a (meth)acrylate is prepared by transesterification reaction of methyl(meth)acrylate and an alcohol, methyl alcohol which is generated as a by-product is generally removed from the reaction system in order to efficiently prepare the (meth)acrylate. A method for removing methyl alcohol which is generated as a by-product during the preparation of the (meth)acrylate is generally classified into the following two methods:
The first method is a method which includes using a reactor equipped with a distillation column when the transesterification reaction of methyl(meth)acrylate and an alcohol is carried out, forming an azeotropic mixture of methyl(meth)acrylate and methyl alcohol, and taking out the resulting azeotropic mixture from the overhead of the distillation column (see, for example, working example 4 of Patent Literature 11). The azeotropic mixture obtained by this method is generally used as a raw material for preparing methyl methacrylate in order to effectively use the azeotropic mixture. When the above-mentioned azeotropic mixture is used as a raw material for preparing(meth)acrylate, methyl(meth)acrylate is effectively used as a raw material. However, methyl alcohol which is a by-product has a defect such that the methyl alcohol hinders the transesterification reaction of methyl(meth)acrylate and an alcohol.
The second method is a method which includes using a reactor equipped with a distillation column when the transesterification reaction of methyl(meth)acrylate and an alcohol is carried out, adding a hydrocarbon compound which forms an azeotropic mixture with methyl alcohol at a temperature lower than the boiling point of methyl(meth)acrylate to a reaction system of methyl(meth)acrylate and an alcohol, and discharging the azeotropic mixture of methyl alcohol and the hydrocarbon compound at an azeotropic temperature of methyl alcohol and the hydrocarbon compound from the overhead of the distillation column (see, for example, working example 5 of Patent Literature 9). In this method, a preferred reaction solvent includes aliphatic saturated hydrocarbon compounds having 5 to 8 carbon atoms, such as n-pentane, n-hexane, n-heptane, n-octane, 2,3-dimethylbutane, 2,5-dimethylhexane and 2,2,4-trimethylpentane, and the like (see, for example, columns [0017]-[0018] of Patent Literature 9). According to the second method, for example, when n-hexane is used as a reaction solvent, methyl(meth)acrylate which is a raw material is included in the azeotropic mixture other than n-hexane and methyl alcohol, which is discharged from the overhead of the distillation column. However, it is difficult to separate the methyl(meth)acrylate from n-hexane and methyl alcohol. Therefore, according to the second method, the azeotropic mixture is treated as a waste, fuel and the like, nevertheless the azeotropic mixture includes methyl(meth)acrylate which is useful as a raw material of a (meth)acrylate.
As a method for effectively using the azeotropic mixture obtained in the second method, it is thought that a part of the distillate which is distilled from the overhead of the distillation column is recirculated to the overhead of the distillation column, water is added to the remaining distillate to separate the distillate into two layers, supplying the upper layer of the two layers, which mainly contains a reaction solvent, to the middle portion of the distillation column, and the lower layer of the two layers, which mainly contains methyl alcohol and water, is disposed as a waste (see, for example, claim 1 of Patent Literature 2). According to the second method, however, it is difficult to extract methyl(meth)acrylate only to the upper layer, and methyl(meth)acrylate is included in the lower layer. Therefore, methyl(meth)acrylate contained in the lower layer is disposed as a waste in the end.